Why do floodplains and levees form naturally, and why does human interference with them worsen floods?

Floodplains and levees are the natural products of a river doing what rivers do in their lower course, losing energy and depositing load. Treating them as free land is what converts a routine flood into a disaster.

How they form naturally

• Overbank deposition: When a river exceeds bankfull stage, water spreads sideways, loses velocity abruptly and drops its load.
• Sorting builds the levee: The coarsest sand and silt settle first, right at the channel edge, raising a natural embankment; finer clay travels further and builds the flat floodplain behind it.
• Lateral migration: Meander cut-and-fill reworks sediment across the valley floor, as in the Kosi and Gandak plains.
• A built-in safety valve: The floodplain is the channel's overflow reservoir, storing water and shaving the flood peak.

Why human interference worsens floods

• Occupation of the storage space: Settlement, cropping and urban expansion remove the very area meant to hold surplus water.
• Artificial embankments raise the bed: Confining the river forces deposition inside the channel, so the bed rises and the river eventually runs above the plain; the Kosi embankment breach of 2008 is the classic case.
• Loss of infiltration: Paving and drainage of wetlands cut absorption and speed up runoff.
• Breach floods are catastrophic: A failed embankment releases water suddenly onto a false sense of security.

The floodplain is not land the river has abandoned. It is land the river uses occasionally, and building on it converts an expected event into an avoidable catastrophe.